X-ray Detectors in Space First To Witness New Supernovae

The X-ray detectors present in space could be the first ones to witness the new supernovae that signal the death of massive stars, says a team of astronomers from the University of Leicester.

The X-ray radiation in the first few minutes of collapsing massive stars has been measured by the astronomers and they consider this as the key factor of the supernova shock wave that first escapes from the star.

These new findings surprised Dr Rhaana Starling of the University of Leicester's Department of Physics and Astronomy.

Dr Starling said: "The most massive stars can be tens to a hundred times larger than the Sun. When one of these giants runs out of hydrogen gas it collapses catastrophically and explodes as a supernova, blowing off its outer layers which enrich the Universe. But this is no ordinary supernova; in the explosion narrowly confined streams of material are forced out of the poles of the star at almost the speed of light. These so-called relativistic jets give rise to brief flashes of energetic gamma-radiation called gamma-ray bursts, which are picked up by monitoring instruments in Space that in turn alert astronomers."

Gamma-ray bursts are known to arise in stellar deaths because coincident supernovae are seen with ground-based optical telescopes about 10 to 20 days after the high energy flash. Most often when the star's surface reacts to the core collapse, often termed the supernova shock breakout, it is missed.

The energetic supernovae go hand-in-hand with gamma-ray bursts, but for this sub-class it may be possible to identify X-ray emission signatures of the supernova in its infancy.

The X-ray detectors that the researchers have used in this study were built partly in the UK at the University of Leicester and are on the X-ray telescope on-board the Swift satellite.

Data from Swift of a number of gamma-ray bursts with visible supernovae have shown an excess in X-rays received compared to expectations. This excess is thermal emission, also known as blackbody radiation.

Dr Starling added: "We were surprised to find thermal X-rays coming from a gamma-ray burst, and even more surprising is that all confirmed cases so far are those with a secure supernova identification from optical data. This phenomenon is only seen during the first thousand seconds of an event, and it is challenging to distinguish it from X-ray emission solely from the gamma-ray burst jet. That is why astronomers have not routinely observed this before, and only a small subset of the 700+ bursts we detect with Swift show it."

"It all hangs on the positive identification of the extra X-ray radiation as directly emerging from the supernova shock front, rather than from the relativistic jets or central black hole. If this radiation turns out to be from the central black-hole-powered engine of the gamma-ray burst instead, it will still be a very illuminating result for gamma-ray burst physics, but the strong association with supernovae is tantalizing."

The supernovae at their visible-light peak will be viewed by astronomers when they are already some days old. But for those that are most active will be visible the very moment they are born, through X-ray eyes.

The study was published in the Monthly Notices of the Royal Astronomical Society.